Plasma thrusters are known which comprise a plasma chamber with an anode and a cathode which set up an electic field in the chamber, the cathode acting as a source of electrons. Magnets provide regions of high magnetic field in the chamber. A propellant, typicaly a noble gas, is introduced into the chamber. Electrons from the cathode are accelerated through the chamber, ionizing the propellant to form a plasma. Positive ions in the plasma are accelerated towards the cathode, which is at an open end of the chamber, while electons are deflected and captured by the magnetic field, because of their higher charge/mass ratio. As more propellant is fed into the chamber the primary electrons from the cathode and the secondary electrons from the ionization process continue to ionize the propellant, projecting a continuous stream of ions from the open end of the thruster to produce thrust.
Examples of multi-stage plasma thrusters are described in US2003/0048053, and divergent cusped field (DCF) thrusters are also known.